This invention relates to apparatus for detecting leaks in nuclear fuel elements and particularly to leak detection apparatus having a gas detection mechanism.
In many nuclear reactor designs, the reactor vessel has an inlet and an outlet for circulation of a coolant in heat transfer relationship with a core contained therein that produces heat. The core comprises an array or arrays of fuel assemblies which contain fuel elements. The fuel element is generally a cylindrical metallic sheath sealed at both ends containing nuclear fuel and internally pressurized with a gas such as helium. The nuclear fuel which may be, for example, ceramic fuel pellets of a uranium compound is stacked in the fuel elements. During reactor operation, the nuclear fuel pellets fission thereby generating heat in a manner well known in the art. The reactor coolant absorbs the heat while circulating through the core thereby cooling the fuel elements of the core and heating the coolant. Of course, the heated coolant may then be used to produce electricity in a conventional manner.
There are several methods known in the art for loading uranium fuel pellets into the cylindrical metallic sheath for use in a light water reactor. One such method comprises simply placing the fuel pellets in the metallic sheath by hand. This method is possible because non-irradiated uranium fuel does not pose serious radiological problems to working personnel. However, when the nuclear fuel utilized is of a more toxic nature such as plutonium or a reprocessed uranium compound, then increased safeguards must be employed to prevent releasing radioactive contaminants to the atmosphere and to prevent overexposing working personnel. When such toxic fuel is employed, it is known in the art to use glove box handling techniques to load the fuel pellets into the metallic sheath. Furthermore, when such toxic fuel is used the entire fabrication building must be constructed to strict design requirements that enable the building to withstand environmental hazards without releasing airborne contaminants or other radiological contaminants from the building. Such environmental hazards that the building must withstand include seismic disturbances, high wind loads, overpressure loads, and missiles that become airborne due to high winds.
In addition to being able to withstand environmental disturbances, the fabrication building must be constructed with a passageway to enable the completed fuel element to be tested for leaks and removed from the building without releasing contaminants to the atmosphere. The ability to test fuel elements for leaks as the fuel elements exit the building increases the probability that no leaks will have developed between the time of testing and exiting the building, which minimizes the risk of releasing contaminants to the atmosphere by means of a defective fuel element.
An example of an arrangement for transferring nuclear fuel elements through the walls of a nuclear installation is described in U.S. Pat. No. 3,711,993 to J. Liesch et al. issued Jan. 23, 1973. The Liesch patent disclosed a cylindrical chamber disposed in an isolation wall between a contaminated and an uncontaminated area for providing a passageway therebetween. The chamber is enclosed between a pair of shutters with variable apertures that allow the passage of a nuclear fuel element therethrough. The chamber may be flushed or purged with a gas so as to entrain contaminated particles in the gas which may then be conducted through a filter located remote from the chamber thereby trapping the contaminated particles in the filter. The patent to Liesch indicates that such a filter may be of the electrostatic or activated carbon type. While the Liesch patent describes a particular arrangement for transferring a nuclear fuel element through a wall of a nuclear installation, it does not solve the problem of testing the fuel element for leaks before removing the fuel element from a structure capable of withstanding environmental hazards.